Starter is located opposite side to the carbs, under the exhaust manifold. Positive feed is on top of the starter and case is the earth. Always run a second earth from battery negative to motor cases.
Items that tend to fail in this area are bendix, starter and positive / negative leads.
I'd be astonished if the pump isn't seized - the wear ring expands due to salt corrosion pressing the liner against the prop blades - this will need stripping and a new liner installing by a machine / Jetski shop. If this is indeed seized, the engine won't turn over - pull the pump, it's only 8x bolts and try again - your starter may well be fine.
All the running gear is based on a Yamaha Superjet by the way